1. Field of the Invention
The present invention relates to a howling detecting and suppressing apparatus for, a howling detecting and suppressing method of, and a howling detecting and suppressing computer program product for automatically detecting and suppressing howling sound components occurred as a result of acoustic coupling between a speaker and a microphone, and an acoustic apparatus comprising the same.
2. Description of the Related Art
Up until now, there have been proposed a wide variety of howling detecting and suppressing apparatuses for automatically detecting and suppressing howling sound components occurred as a result of acoustic coupling between a speaker and a microphone. One of the conventional howling detecting and suppressing apparatuses of this type is disclosed, for example, in the Patent Application Laid-Open No. H07-143034.
One typical example of the howling detecting and suppressing apparatus will be described hereinlater with reference to FIG. 19. The conventional howling detecting and suppressing apparatus is shown in FIG. 19 as comprising an input terminal 1901, an A/D converter 1902, a plurality of notch filters 1903, a plurality of coefficient memories 1904, a D/A converter 1905, an output terminal 1906, a fast Fourier Transformation performing unit 1907, a judging unit 1908, a coefficient selecting means 1909, and a coefficient memory 1910. In the conventional howling detecting and suppressing apparatus, the input terminal 1901 connected with, for example, a microphone, not shown, is adapted to input an analog sound signal therethrough. The A/D converter 1902 is adapted to convert the analog sound signal inputted through by the input terminal 1901 into a digital sound signal including a plurality of frequency segments. The notch filters 1903 are connected in series with the A/D converter 1902. Each of the notch filters 1903 uniquely corresponds to a frequency segment and is adapted to filter the corresponding frequency segment of the digital sound signal in accordance with a coefficient stored in one of the coefficient memories 904 to suppress or pass through the corresponding frequency segment. Each of the coefficient memories 1904 uniquely connected to one of the notch filters 1903 and is adapted to store the coefficient to be used by the one of corresponding notch filters 1903. D/A converter 1905 is adapted to convert the digital sound signal including the frequency segments thus filtered through by all of the notch filters 1903 into a filtered analog sound signal. The output terminal 1906 is adapted to output the analog sound signal thus filtered to, for example, a speaker, not shown. The fast Fourier Transformation performing unit 1907 is adapted to analyze the frequency segments of the digital sound signal filtered through by all of the notch filters 1903 to generate analyzed frequency segment information. The judging unit 1908 is adapted to detect a peak frequency segment to be used to suppress and eliminate a howling sound component. The coefficient memory 1910 is adapted to store coefficients such as, for example, coefficients f0, f1, f2, . . . fn to be set to the notch filters 1903. The coefficient selecting means 1909 is adapted to select the coefficients to be respectively set to the notch filters 1903 on the basis of the peak frequency segment detected by the judging unit 1908 from among the coefficients f0, f1, f2, . . . fn stored in the coefficient memory 1910.
The operation of the conventional howling detecting and suppressing apparatus above stated will be described hereinlater. In the conventional howling detecting and suppressing apparatus, each of the notch filters 1903 is assumed to have flat frequency characteristics in their respective default states.
In the conventional howling detecting and suppressing apparatus, the input terminal 1901 connected with, for example, a microphone, not shown is operated to input an analog sound signal therethrough. The A/D converter 1902 is operated to convert the analog sound signal inputted through by the input terminal 1901 into a digital sound signal including a plurality of frequency segments. The notch filters 1903 connected in series with the A/D converter 1902 are operated to input the digital sound signal. Each of the notch filters 1903 is operated to filter one of the frequency segments of the digital sound signal in accordance with a coefficient stored in the corresponding one of the coefficient memories 1904 to suppress or pass through the one of the frequency segments of the corresponding frequency segment. The D/A converter 1905 is operated to convert the digital sound signal including a plurality of frequency segments thus filtered through by all of the notch filters 1903 into a filtered analog sound signal. The output terminal 1906 is operated to output the filtered analog sound signal to, for example, a speaker, not shown. The fast Fourier Transformation performing unit 1907 is operated to analyze the frequency segments of the digital sound signal filtered through by all of the notch filters 1903 by calculating power values of frequency segments to generate analyzed frequency segment information. The judging unit 1908 is operated to judge maximum and average power values of frequency segments to detect a maximum frequency segment on the basis of the analyzed frequency segment information generated by the fast Fourier Transformation performing unit 1907. Here, a maximum frequency segment is intended to mean a frequency segment having the maximum power value. A frequency segment having the maximum power value may also be referred to as a peak frequency segment.
The analog sound signal inputted through the input terminal 1901, for example, includes a howling sound component; the frequency segment containing the howling sound component will appear as a peak frequency segment because of the fact that the frequency segments containing the howling sound component have great power values. This means that the judging unit 1908 can detect a frequency segment containing a howling sound component as a maximum frequency segment.
More specifically, the judging unit 1908 is operated to judge maximum and average power values of frequency segments to detect a maximum frequency segment, i.e, a peak frequency segment, and judge if the ratio of the maximum power value to the average power value is greater than a predetermined threshold value or not. The judging unit 1908 is operated to determine that the maximum frequency segment contains a howling sound component if it is judged that the ratio of the maximum power value to the average power value is greater than the predetermined threshold value because of the fact that a frequency segment containing a howling sound component has a peak power value. Alternatively, the judging unit 1908 may count how many times it is judged that the ratio of the maximum power value to the average power value with respect to a maximum frequency segment is greater than the predetermined threshold value and determine that the maximum frequency segment contains a howling sound component if the number of times thus counted with respect to the maximum frequency segment exceeds a predetermined number because of the fact that the frequency segments containing howling sound components continuously maintain remarkably great power values. This means that the conventional howling detecting and suppressing apparatus thus constructed detects a howling frequency segment by judging whether the ratio of the maximum power value to the average power value is greater than a predetermined threshold value or not because of the fact that a frequency segment containing a frequency component has a peak power value.
The judging unit 1908 is operated to generate and transmit howling information indicating the maximum frequency segment thus determined to contain a howling sound component to the coefficient selecting means 1909. The coefficient selecting means 1909 is operated to select a coefficient specified for the howling frequency segment, for example, coefficient f0, to be set to one of the notch filters 1903 corresponding to the howling frequency segment from among the coefficient f0, f1, f2, . . . fn stored in the coefficient memory 1910. The coefficient selecting means 1909 is operated to transfer the thus selected coefficient f0 stored in the coefficient memory 1910 to the corresponding one of the coefficient memories 1904 uniquely connected to the one of the notch filters 1903 corresponding to the howling frequency segment. The one of the notch filters 1903 corresponding to the howling frequency segment is operated to filter the howling frequency segment in accordance with the coefficient f0 stored in the corresponding one of the coefficient memories 1904 to suppress and eliminate the howling sound component.
The conventional howling detecting and suppressing apparatus above described detects a howling frequency segment by judging whether the ratio of the maximum power value to the average power value is greater than a predetermined threshold value or not, making it possible to automatically and reliably detect the howling sound component regardless of whether the noise level of the inputted sound signal fluctuates.
Furthermore, the conventional howling detecting and suppressing apparatus above described comprises a plurality of notch filters 1903 each corresponding to a frequency segment to filter the corresponding frequency segment of the digital sound signal in accordance with a coefficient stored in the corresponding one of the coefficient memories 1904 to suppress or pass through the corresponding frequency segment, thereby enabling to automatically and reliably suppress the howling sound component.
The conventional howling detecting and suppressing apparatus, however, encounters a drawback that the conventional howling detecting and suppressing apparatus may erroneously detect a howling frequency segment when the conventional howling detecting and suppressing apparatus happens to input a sound signal containing a frequency segment with a remarkably great power value. This means that the conventional howling detecting and suppressing apparatus may erroneously detect a howling frequency segment when the power value of the frequency segment contained in the sound signal is remarkably great because of the fact that the conventional howling detecting and suppressing apparatus detects a howling sound component on the basis of the ratio of the maximum power value to the average power value.
The conventional howling detecting and suppressing apparatus, furthermore, encounters another drawback that the conventional howling detecting and suppressing must increase the number of the notch filters 1903 and coefficients f1 to fn in order to enhance the frequency resolution because of the fact that the conventional howling detecting and suppressing apparatus must comprise the number of notch filters 1903 equal to the number of frequency segments to be filtered. This means that the conventional howling detecting and suppressing apparatus is required to be large in the size in order to enhance the frequency resolution.
The present invention contemplates resolution of such problems.